This invention relates to methods of quenching heated metallic objects.
It is very well known that quenching a metallic object (i.e., rapidly chilling the object from a heat treatment temperature in the austenitic range to a much lower, usually room, temperature) can significantly improve its mechanical properties and characteristics. Quenching is used to harden the object and/or to improve its mechanical properties, by controlling internal crystallisation and/or precipitation, for example. Traditionally, quenching has been carried out using liquids such as water, oil or brine, either in the form of an immersion bath or a spraying system. In more recent years, gas quenching methods have been developed. Gas quenching has the advantages of being clean, non-toxic and leaving no residues to be removed after quenching, however difficulties have been encountered in achieving similarly high quenching rates as are provided by more conventional liquid quenching processes.
Quenching is a high speed process, requiring the heat within the object to be drawn away at a high heat flow density through the cooled surface of the object. It is usually desirable for the quenching of the object to be uniform, so that the quenched object has uniform surface or internal characteristics, however, uniformity of quenching is difficult to achieve in most quenching techniques, due to various factors, principally Leidenfrost""s phenomenon. The quenching effect of any quench system is usually characterised in terms of the Grossman quench severity factor, H; for liquid quenchants such as water or oil, H usually falls in the range 0.2 to 4. Such high values of H are not easily attainable using gas quenching; when quenching using gas, the cooling intensity can be increased using several different means; increasing the quenching pressure; increasing the velocity at which the gas is sprayed on to the object; choice of gas (nitrogen is less preferable than helium, which is less preferable than hydrogen, because of their respective heat transfer coefficients, although helium and hydrogen are expensive compared to nitrogen); optimising the gas flow conditions and enhancing the turbulence, and enhancing the cooling of the gas.
Gas quenching employing multiple cooling gas streams comprising mainly nitrogen, argon and/or helium at pressures up to 60 bar has been practised in vacuum furnaces, and its characteristics for quenching bulk components are well known. More recently the gas quenching of single or small groups of components which had been heated in either vacuum or conventional atmosphere furnaces has been proposed. To eliminate the need to cool the furnace structure, these techniques involve the transfer of the object to be quenched to a specially designed cold chamber, as is known in the art.
In order to meet the criteria for uniform quenching of a single object or component it is necessary for the quenchant to reach the surface of the object uniformly. In practical gas quenching processes this implies that gas which has been heated through contact with the object must also leave the surface uniformly (so that further fresh, cold gas can reach the surface to continue the quenching process); therefore discrete amounts of arriving and departing gas must exist. Theoretically these amounts would ideally be infinitely small, but practical considerations necessitate that they be as large as possible so far as is consistent with substantially uniform heat transfer.
A second factor affecting quenching uniformity is the interaction of the individual gas streams. It has been shown that, for constant mass flow and a stream width (d) to distance between the gas nozzle orifice and the surface of the object (a) ratio of four, the heat transfer coefficient reaches a maximum when the distance between adjacent gas streams (b) is three times the stream width (d). The turbulence formed at the edges of the gas streams as they impinge on the object surface is known to have a significant effect on the transfer of heat, however the form and size of these turbulent areas is difficult to predict due to the complex interaction between the gas streams.
A further factor affecting the uniformity of gas quenching is that although the velocity of the gas striking the object surface should be as high as possible, and as near perpendicular to the surface as possible, the velocity and angle of incidence relative to the surface of the gas streams must also be as uniform as possible, as the heat transfer coefficient is dependent on both of these. It has been suggested that, to maximise the heat transfer coefficient and to minimise the interaction factor between adjacent gas streams, the distance (a) between the gas nozzle orifice and the surface should be as large as possible so far as is consistent with the loss of velocity of the gas stream over distance. For example, U.S. Pat. No. 5,452,882 proposes that, in order to achieve a quench severity factor, H, of between 0.2 and 4, a plurality of gas streams of diameter d should be directed towards the object to be quenched from nozzles (of diameter d) spaced at a distance between 2 d and 8 d from the surface of the object and with a distance between adjacent nozzles, b, of between 4 d and 8 d. There is a continuing need to provide an efficient and economic gas quenching process capable of high quench severity and of substantial uniformity.
Accordingly, the present invention provides a method of quenching a heated metallic object comprising discharging a plurality of discrete gas streams from a plurality of nozzle outlets such that the gas streams impinge substantially uniformly over the outer surface of the object, wherein the distance (a) between each nozzle outlet and the outer surface of the object against which the associated gas stream impinges is less than or equal to half the diameter (d) of the nozzle outlets.
For the avoidance of doubt it should not be inferred from the use of the word xe2x80x9cdiameterxe2x80x9d that the invention is limited to gas streams of circular cross section; the present invention extends to gas streams of any cross-sectional shape, the xe2x80x9cdiameterxe2x80x9d of these being calculated through assuming that the cross-sectional area of a non-circular gas stream, for the purpose of putting this invention in to practice, is in fact circular. Thus the word xe2x80x9cdiameterxe2x80x9d where used herein should be interpreted as meaning the diameter of a circular gas stream or the theoretical diameter of a circular gas stream which has an equal cross-sectional area to a non-circular stream. For such small distances between nozzle outlet and the object, the cross-sectional area and the xe2x80x9cdiameterxe2x80x9d of the gas stream remains substantially constant throughout its transit between nozzle outlet and the object, and equal to the cross-sectional area and the xe2x80x9cdiameterxe2x80x9d of the nozzle outlet.
The nozzle outlets may be of substantially equal cross-sectional area, or the area of the nozzles may vary, provided that the total area of nozzles per unit area of the object to be cooled remains substantially constant. It may, for example, be advantageous to have different nozzle areas in order to quench an object having a complex or convoluted surface shape or configuration.
We have discovered from investigating the complex interaction of the gas streams that there is an unexpected and surprisingly large and rapid increase in the heat transfer rate at very small values of the distance between the gas stream nozzle outlet and the surface of the object (ie where axe2x89xa60.5 d), when the areas of high turbulence produced at the edges of the nozzles interact with the surface of the object to maximise the transfer of heat to the gas and to produce more uniform cooling. Also, as will be described further below, a method in accordance with the invention is demonstrably capable of providing a substantially uniform quench, as a varied quench, as desired.
The method of the invention also enables quench rates to be achieved which are equivalent to conventional oil quenching using nitrogen, without requiring a high pressure quenching environment as is often conventional practice. By mixing hydrogen in to the quenching gas stream quench rates equivalent to those of water quenching can be expected (hydrogen having roughly three times the cooling effect of nitrogen). Adding hydrogen would have a further advantage of keeping the component bright during the quenching process (but at a higher gas cost than nitrogen alone).
There are further practical advantages arising from the use of such small distances between the gas nozzle outlet and the object surface. As this distance (a) decreases, the pressure necessary to supply the gas streams at the required velocity will increase; to generate such pressures using conventional compressor apparatus (as suggested in U.S. Pat. No. 5,452,882, for example) is difficult and costlyxe2x80x94both in capital and running costsxe2x80x94but if the gas streams were supplied from a compressed or liquid gas source there would be no need for compressor apparatus. Instead, the gas source would provide high pressure gas, the pressure of which could be easily and cheaply regulated down if necessary, so that there would be no compression cost (gases such as nitrogen routinely being supplied at high pressure, or in liquid form), the only cost therefore being that of the gas. Even the gas cost need not necessarily be totally lost, as the cold wall quenching chamber could be run at a small excess pressure over ambient, 10 kPa say, and the quenching gas reflected from the object used as the entire heat treatment protective atmosphere, or part thereof.
Preferably the distance (b) between adjacent nozzle outlets is less than or equal to eight times the diameter (d) of the nozzle outlets, and preferably more than two times this distance (d), so as to ensure uniformity of quenching.
The gas streams are preferably directed so as to impinge substantially perpendicularly on the surface of the object, to maximise quench severity.
Because the rate of cooling during quenching is directly related to the velocity of the gas streams, and the velocity to the gas supply pressure, it is a relatively simple matter to control the cooling rate. Those skilled in the art will appreciate the appropriate means whereby the gas supply pressure to the nozzle outlets can be controlled, thereby to achieve a very accurately controllable rate of cooling during the quenching process; it is patently possible to produce any instantaneous cooling rate, within the limit of the maximum cooling rate possible, so that austempering and marquenching of objects are easily achievable. Moreover, because the method of the invention is primarily intended for the quenching of single objects, it is possible to control with a high degree of accuracy the quenching rate with respect to the surface area of the object (so as, for example, to marquench one area of component whilst fast oil quenching another area in a single operation) and/or with respect to the quenching cycle (so as to vary the quenching rate during the quench), by controlling appropriately the quench gas flow rate, pressure and/or composition, and/or by varying the quench gas flow rate between different nozzles.